1. Field of the Invention
The present invention generally relates to image forming apparatuses.
2. Description of the Related Art
In conventional image forming apparatuses of the electrophotography type, a switch-back system and a one-pass system are known, both referring to different methods for forming an image on both sides of a transfer medium, such as a sheet of paper, “copy paper”, and so on.
In the switch-back system, first an image is formed on one side (such as the front side) of the transfer medium by a transfer unit, and then the transferred image is fused on the transfer medium by a fusing unit. The transfer medium is then inverted and switched back so that another image can be transferred and fused on the other side (such as the back side) of the transfer medium.
In the one-pass system, after an image is transferred onto both sides of a transfer medium by a double-side transfer unit, the transfer medium is passed through the fusing unit. Thus the image is formed on both sides of the transfer medium without the transfer medium being inverted and switched back.
The one-pass system is superior to the switch-back system in the following respects. For one thing, the one-pass system does not require the complex switch-back mechanism for inverting and switching back the transfer medium, with the associated increase in cost. For another, the one-pass system can avoid the long time required by the switch-back system for image formation. For still another, the one-pass system can avoid the jamming of the transfer medium that may occur when the transfer medium is curled by the heat from the initial fusing unit before it is switched back for printing the back side.
Examples of such image forming apparatus of the one-pass type for double-sided printing that do not require an inverting mechanism or a double-sided transport path are disclosed in the following documents.    Patent Document 1: Japanese Laid-Open Patent Application No. 2002-189358    Patent Document 2: Japanese Laid-Open Patent Application No. 2-259670    Patent Document 3: Japanese Laid-Open Patent Application No. 9-211900    Patent Document 1 discusses an image forming apparatus with a first and a second intermediate transfer belt. The second intermediate transfer belt is capable of transporting a transfer medium, such as a printing paper, and also transferring a toner image onto the first intermediate transfer belt.
Patent Document 2 discusses an image forming apparatus that transfers a toner image onto both sides of a transfer paper using a pair of photosensitive drums for individually forming toner images having mutually different charge polarities.
Patent Document 3 discusses an image forming apparatus that forms a toner image on both sides of a transfer paper using individual intermediate transfer belts for retaining a back surface image of individual colors, and a contacting/separating mechanism for bringing each transfer belt into and out of contact with a photosensitive drum.
These image forming apparatuses of the one-pass type, however, still have room for improvement in image formation speed, size, price, image quality, and so on.
For example, in the image forming apparatus according to Patent Document 1, after a toner image for the back surface is transferred onto the second intermediate transfer belt, a transfer medium is transported on the second intermediate transfer belt with the transferred toner image thereon. As a result, the transfer medium may rub against the toner image of the back surface on the intermediate transfer belt, thus causing an image quality defect. Further, the toner image needs to be transported for the lengths of the two intermediate transfer belts before the image is transferred onto the back surface of the transfer medium, resulting in a long print time.
The image forming apparatus according to Patent Document 2 requires a pair of photosensitive drums and developing units for forming a toner image on each of the front and back surfaces, with the resultant increase in the number of components, cost, and size of the apparatus.
The image forming apparatus according to Patent Document 3 requires four intermediate transfer belts for the colors of K (black), M (magenta), C (cyan), and Y (yellow), with an increase in the number of components, cost, and size of the apparatus. In addition, color matching adjustment is difficult.